Rotary encoders, especially optical rotary encoders, have found widespread use in the industry for determining the rotational position of motor shafts, gearing, axles and the like, hereinafter collectively referred to as "object shaft."
In the typical optical encoder, a transparent disc is rotated by the object shaft for which position information is desired. The code disc includes a large number of radially-positioned indicia which are coded to indicate the particular radial position of each indicia on the code disc. A light or illumination source is provided on one side of the code disc to project a beam of light through the disc and onto a photo detector. As the code disc is rotated by the object shaft, the indicia on the disc modify the light beam as the indicia pass therethrough, thereby providing a unique light pattern to the photo detector which can decode the pattern to indicate the position of the disc. This information is then processed and provided to the user by other circuitry within the encoder.
The code disc is coupled to the object shaft by a coupling member. Typically, this coupling member takes the form of a shaft which is supported for rotation within the encoder and which is brought outside of the encoder housing for attachment to the object shaft.
This is called the "protruding shaft" style of rotary encoder. The other commonly available rotary encoder is called the "kit" style. With the "kit" style, the object shaft is used as an integral part of the encoder. The object shaft is inserted through the encoder and the code disc and is then coupled to the code disc via a flexible coupling.
With respect to the "kit" encoders, problems associated with such arrangements include shaft end play, which makes precise alignment of the code disc and the optics difficult. Also a problem is the requirement that the user handle and adjust key optical and electronic components. Since, in the "kit" style, the object shaft becomes an integral part of the encoder, the user necessarily must have access to the interior of the encoder, thereby exposing delicate portions of the encoder to potential damage. Additionally, "kit" encoders have used a single bearing to support the shaft of interest, thereby permitting excessive shaft movement and hence further misalignment of the code disc and the optics.
With respect to the "protruding shaft" configuration, the inherent disadvantage is its high profile. The shaft which protrudes from the encoder must extend a sufficient distance from the bottom of the encoder to permit a flexible coupling to be mounted thereto. Typically, this distance should be as great as the diameter of the shaft. The other end of the flexible coupling is then connected to the object shaft. Thus, the length of the encoder is extended beyond the encoder housing by at least the length of the flexible coupling used. This additional length is often intolerable, especially in situations in which space is at a premium. Additionally, the protruding shaft represents a long moment arm with respect to the structures which support the shaft within the encoder itself, thus the amount of force which the user can exert upon the shaft, and, hence, upon the bearings and other support structures within the encoder, can vary over a wide range. This potential variation requires that at least two bearing structures be utilized within the encoder and that these bearing structures have a substantial spacing between one another to provide the desired "stiffness" to the shaft. This spacing between the bearings also adds to the height profile of the encoder.